1. Field of the Invention
The present invention generally relates to a method for fabricating a substrate, in particular, to a method for fabricating a blind via structure of a substrate.
2. Description of Related Art
Currently, in the field of circuit board technology, a multi-layer circuit board usually has a plurality of conductive blind via structures. Through the conductive blind via structures, multiple layers of circuits in the multi-layer circuit board may be electrically connected between neighboring layers. In order to meet the recent developing trend of circuit boards that a line width and a line pitch are reduced and a line density is increased, the conductive blind via structures are usually formed through a laser drilling process.
FIGS. 1A to 1E are schematic cross-sectional views of a flow of a conventional laser drilling process. Referring to FIG. 1A, first, a circuit layer 11 is formed on a circuit board 10. Next, referring to FIG. 1B, an insulating layer 12 is formed on the circuit layer 11 and a copper foil layer 13 is formed on the insulating layer 12, in which the insulating layer 12 covers the circuit layer 11 and a portion of the circuit board 10. Next, referring to FIG. 1C, a surface uniform etching process (SUEP) is performed on the copper foil layer 13, so as to form a thinned copper foil layer 13′. Next, referring to FIG. 1D, a rough oxide layer 14 is formed on the thinned copper foil layer 13′, in which the rough oxide layer 14 is made of, for example, copper oxide. Then, referring to FIG. 1E, a laser beam L is irradiated, so as to form a blind via 20 passing through the rough oxide layer 14, the copper foil layer 13′, and the insulating layer 12 and linking to the circuit layer 11. Thus, the laser drilling process of the blind via 20 is finished.
After the blind via 20 has been formed, the melting copper may be splashed in the region ablated by the laser, so as to contaminate the via wall, such that it is necessary to clean up the melted copper splashed on the via wall by using an etching process, thereby preventing the via wall from being contaminated to affect the subsequent process. However, the rough oxide layer 14 cannot resist an etchant used in the etching process. That is to say, the rough oxide layer 14 may be corroded by the etchant. When the rough oxide layer 14 is corroded through by the etchant, the copper foil layer 13′, the insulating layer 12, and the circuit layer 11 located under the rough oxide layer 14 may also be corroded by the etchant, thereby affecting the stability of the subsequent process.
In addition, in the flow of the conventional laser drilling process, before forming the rough oxide layer 14, a surface lithographing and etching process is performed on the copper foil layer 13, so as to form the thinned copper foil layer 13′, in which a thickness uniformity of the thinned copper foil layer 13′ may be affected by a surface cleanness of the copper foil layer 13 and a tumbling efficiency of the etchant on the surface of the copper foil layer 13 during the lithographing and etching process. Furthermore, when the rough oxide layer 14 is formed on the surface of the thinned copper foil layer 13′, a portion of copper may be oxidized to form copper oxide. In other words, the thickness uniformity of the thinned copper foil layer 13′ may be unstable due to the fabricating process, which may directly or indirectly affect the quality of the subsequent process for forming the via by using laser.